Work that is in progress and proposed for continuation this year includes the role of temperature and relative humidity in photosensitivity reactions, as well as the role of biologic factors of the host resulting in endogenously produced photosensitizers such as porphyrins. Although exogenous photosensitizing substances are well recognized causes of occupational dermatoses due to their primary irritant ("phototoxic") or allergic ("photoallergic") properties, little is known regarding the mechanisms of their photosensitivity properties on a molecular and submolecular biological level. Our studies this year will extend into this field particularly stressing protein binding sites. In addition to the use of in vitro models and in vivo animal experimentation, our studies will continue to center on the recognition of new adverse photosensitivity reactions in man due to the various compounds which are being introduced into our environment by new developments in industry, manufacturing, and medical practice. Analysis of the phototoxic properties of 8-methoxypsoralen will be extended using tritiated material applied to guinea pig skin. Data will be obtained regarding percutaneous absorption under various environmental conditions using sequential blood levels and concentration remaining in skin. BIBLIOGRAPHIC REFERENCES: Harber, L.C., Bickers, D.R.: The Porphyrias: Basic Science Aspects, Clinical Diagnosis and Management. Year Book of Dermatology 1976. DeLeo, V.A., Poh-Fitzpatrick, M., Harber, L.C., Mathews-Roth, M.: Erythropoietic Protoporphyria. Am. J. Medicine, January, 1976.